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Glycolysis

We now enter the wonderful topic of Glcolysis , which is my favourite topic , glycolysis consists of 10 enzyme catalysed reactions. The first five reactions of glycolysis are what is referred to as the energy investment phase , since ATP is invested into the reactions , the last five reactions are known as the energy payoff phase since it is here that the ATP used is regenerated with two extra ATP molecules , giving glycolysis a net gain of two ATP per molecule of glucose used. It must be said however that the real stars of glycolysis are the enzyme since they allow the reactions to be fast and energy efficient.

 

1st Reaction

This is the first Priming reaction of glycolysis and the first reaction of the energy investment phase as well. The reaction is irreversible. In this reaction Glucose is converted to Glucose-6-phosphate. The reaction uses an ATP molecule and converts it to ADP , the enzyme which catalyses the reaction is Hexokinase.

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2nd Reaction

This reaction converts Glucose -6-phophate to Fructose-6-phosphate via the enzyme Phosphohexose isomerise.

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3rd Reaction

This reaction is the second of two irreversible reactions which occur in the energy investment phase. This reaction is the second priming reaction and consumes an ATP molecule whoch is converted to ADP , in this reaction Fructose-6-phosphate is converted to Fructose-1.6-Bisphosphate via the enzyme Phospho-fructokinase-1 (PFK-1). It must also be noted that this is the most regulated step of glycolysis.

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4th Reaction

This is the splitting reaction , in this reaction the six carbon molecule Fructose-1.6-bisphosphate is converted to two three carbon molecules , one is called Glyceraldehyde -3-phosphate and the other is called Dihydroxyacetone phosphate. The enzyme which catalyses this reaction is Aldolase.

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5th Reaction

This is the final reaction of the energy investment phase , in this reaction the Dihydroxyacetone phosphate is converted to another molecule of glyceraldehyde-3-phosphate via the enzyme Triose phosphate isomerise. Hence when the energy payoff phase begins , there will be two molecules to glyceraldehydes-3-phosphates to be used.

 

 

ENERGY PAYOFF PHASE

6th reaction

In this , the first of the energy payoff reactions , an inorganic phosphate is used , as well as an NAD+ molecule which is converted to NADH and a proton. In this reaction the two molecules of glyceraldehyde-3-phosphate are converted to two molecules of 1,3-Bisphosphoglycerate via the enzyme Glyceraldehyde-3-phophate dehydrogenase.

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7th Reaction

In this reaction two ATP molecules are generated from two ADP molecules , the reaction converts two molecules of 1,3-bisphosphoglycerate to two molecules of 3-phosphoglycerate , via the enzyme Phophsglycerate kinase.

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8th Reaction

In this reaction the two molecules of 3-phosphoglycerate are converted to two molecules of 2-phophogycerate via the enzyme Phophoglycerate  mutase.

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9th Reaction

In this reaction two molecules of water are removed , the two molecules of 2-phosphoglycerate are converted to two molecules of Phosphophenolpyruvate via the enzyme Enolase.

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10th Reaction

This is the final reaction in glycolysis , it generates two molecules of ATP from two molecules of ADP. In this reaction two molecules of Phosphophenolpyruvate are converted to two molecules of pyruvate via the enzyme Pyruvate kinase. It should be noted that this is the only irreversible reaction in the energy payoff phase of glycolysis.

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It can be seen therefore that since two ATP molecules are used in the energy investment phase and Four ATP molecules are produced in the energy payoff phase , that there is a net gain of two ATP molecules from one glucose molecules from glycolysis.

 

So what happens to pyruvate after it’s produced? What’s the Fate of pyruvate?

Pyruvate has three main fates;

 1) pyruvate under anaerobic conditions to produce ethanol and carbon dioxide , whereby , ,  pyruvate is converted to acetaldehyde via the enzyme pyruvate decarboxylase using cofactors TPP and Mg2+ ions  , this reaction produces carbon dioxide as a by product . The acetaldehyde is then converted to ethanol via the enzyme alcohol dehydrogenase , this reaction also converts NADH to NAD+.

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 2) pyruvate under anaerobic conditions to produce lactate ,this reaction occurs via the enzyme lactate dehydrogenase and in this reaction NADH is converted to NAD+. Lactate is produced in vigorously contracting muscles because in such circumstances there is an oxygen debt present and hence anaerobic conditions would be prevalent , and in erythrocytes because they lack mitochondria to produce energy otherwise.

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 3) pyruvate under aerobic conditions to produce carbon dioxide and water. The first stage of this reaction is converting pyruvate to Acetyl-CoA , this reaction occurs via an enzyme complex referred to as the pyruvate dehydrogenase complex , which consists of three enzymes , this reaction produces carbon dioxide and uses CoA-SH as a cofactor , in the reaction NAD+ is converted to NADH. The second stage of the conversion occurs via the citric acid cycle where the Acetyl-CoA is converted to carbon dioxide and water.

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It must also be noted that Fermentation ie. Fates 1 and 2 are used to regenerate NAD+ to be used in glycolysis.

 

 

 References:

Pictures taken from

1)      The BIOCHEMJM  Youtube channel

http://www.youtube.com/watch?v=K-NMuq-XIHo

 

2)      Windsor Education Online

http://windsor.edu/Assi_model.htm

 

 

 

 

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Video Review 1 – Glycolysis Part 2: Fates of Pyruvate

Video Review 1 – Glycolysis Part 2: Fates of Pyruvate

http://www.youtube.com/watch?v=K-NMuq-XIHo

This video was taken from the BIOCHEMJM Youtube channel. First of all , I must say that this topic is my favourite topic throughout this course , this video goes into detail about what happens after the pyruvate is produced during glycolysis. Oddly enough looking through youtube  , although a lot of videos regarding Glycolysis were found , videos detailing the use of pyruvate , or the fates of pyruvate were few and far in between , hence from my point of view I would regard this video as the best and most in depth one that I’ve seen  concerning the topic so far. The video has three main points , 1) the use of pyruvate under anaerobic conditions to produce ethanol and carbon dioxide , 2) the use of pyruvate under anaerobic conditions to produce lactate and 3) the use of pyruvate under aerobic conditions to produce carbon dioxide and water. In the first point the video makes it goes into fermentation and lists the importance of fermentation , stating that it replenishes NAD+  for use in glycolysis. The videos goes into the reaction whereby , under anaerobic conditions ,  pyruvate is converted to acetaldehyde via the enzyme pyruvate decarboxylase and the use of the cofactors TPP and Mg2+ ions  , this reaction produces carbon dioxide as a by product . The acetaldehyde is then converted to ethanol via the enzyme alcohol dehydrogenase , this reaction also converts NADH to NAD+. The second point made is the conversion of pyruvate to lactate under anaerobic conditions , the reaction occurs via the enzyme lactate dehydrogenase and in this reaction NADH is converted to NAD+. Lactate is produced in vigorously contracting muscles because in such circumstances there is an oxygen debt present and hence anaerobic conditions would be prevalent , and in erythrocytes because they lack mitochondria to produce energy otherwise. The third point discussed is the conversion of pyruvate to water and carbon dioxide under aerobic conditions. The first stage of this reaction is converting pyruvate to Acetyl-CoA , this reaction occurs via an enzyme complex referred to as the pyruvate dehydrogenase complex , which consists of three enzymes , this reaction produces carbon dioxide and uses CoA-SH as a cofactor , it was also noted in the video that in the reaction NAD+ is converted to NADH. The second stage of the conversion occurs via the citric acid cycle where the Acetyl-CoA is converted to carbon dioxide and water. Overall I must say that I enjoyed the video for a number of reasons ,such as ,  the length , the video is roughly about eighteen minutes long , this is GOOD  because it allows a maximum of information to be absorbed in a smaller period of time , it also means that if one were to want to re watch the video , it wouldn’t seem like such a task , also what I have found is that because the download size of the video is relatively small and would not take up much memory , people are more inclined to put the video on their phone and watch in their free time to refresh themselves on the topic , this is what I personally do. Another reason I like this video is because after watching this video and attempting an online quiz I scored 94.5% , which is also GOOD  , I think that the fates of pyruvate is an important topic since it explains why glycolysis even occurs to produce pyruvate in the first place. If I had to make a recommendation to better the video , I would include more dynamic slides or video clips , since the lecture is in a video format it allows moving images and flashy effects to be used , I think such inclusions would better help to keep the attention of the viewer.

In conclusion , the video is extremely informative , well produced , formatted nicely , it is interesting , flows one point into the other and is overall a great resource for anyone interested in glycolysis or biochemistry.